Magnetic resonance scanners are imaging devices which, in order to image an investigated object, align nuclear spins of the investigated object with a strong external magnetic field and stimulate this alignment by a magnetic alternating field for precession. The precession and/or return of the spin from this stimulated state into a state with lower energy in turn generates an alternating magnetic field in response, which is received by way of antennae.
With the aid of magnetic gradient fields, spatial encoding is imprinted on the signals, subsequently enabling an assignment of the received signal to a volume element. The received signal is then evaluated and three-dimensional imaging of the investigated object provided. Local antennae, so-called local coils, which are arranged directly on the investigated object to obtain a better signal to noise ratio, may be used to receive the signal.
To reduce the number of cable connections between the local coil and the magnetic resonance scanner, the signals of several antenna coils may be jointly transmitted by frequency-division multiplexing. Implementation by mixers on various carrier frequencies, also referred to as intermediate frequencies, is required for this. As the frequency and the phase of the signals are of major importance for imaging with magnetic resonance signals, it is also necessary to obtain these variables when transmitting with frequency-division multiplexing. The auxiliary signals for mixing are therefore customarily generated centrally in the magnetic resonance scanner with a high degree of precision and then distributed to the local coils.
German Patent Publication No. DE 10 2008 062 855 A1 describes a system in which a multiplicity of signals is spectrally separated from one another from a base system to a satellite and in the reverse direction and by a bandpass filter bank in each case.
A magnetic resonance scanner is known from German Patent Publication No. DE 10 2009 008 623 A1, for example, in which mixed frequency signals are supplied for frequency-division multiplexing by way of separate cables.
In systems with lower magnetic fields, the Larmor frequencies are lower, such that the frequency schemes of the high field systems may no longer be used to avoid interference by the intermediate frequencies.